Many of the current foggers used for spraying or dispersing liquid particles, such as pesticides, defoliants, fungicides and other chemicals, are known as ultra-low volume (ULV) cold aerosol generators or fog generators. Such devices normally include a prime mover such as a small gasoline-powered engine, an air blower assembly driven by the prime mover, a nozzle assembly, a supply tank for the liquid, and a suitable control means. The liquid is normally fed into the nozzle assembly where it is entrained in the airstream flowing therethrough and is dispersed into the atmosphere as a fog of small droplets. See commonly owned U.S. Pat. No. 4,992,206, herein incorporated by reference.
As the technology for cold aerosol generators has evolved, it has become evident that it is highly desirable to be able to control or effect the amount of air pressure actually delivered from the air blower to the nozzle assembly and thereby control the size or sizes of the liquid droplets being dispensed by the fog generator.
Control of droplet size is critical, for not only environmental and economic considerations, but also because the droplet sizes must comply with the legal regulations for the particular chemical being applied. Legal regulations are promulgated by the United States Environmental Protection Agency and by state and local governments for various chemicals. These regulations typically limit the maximum chemical droplet size in the fog and a chemical's maximum application rate per unit area.
The droplet size is also critical in that its size must be such that it can be properly applied both in terms of its effectiveness and its dispersion range. It has been observed in the art that droplets below a certain size are ineffective because much of these droplets are carried off by the ambient air without engaging the target insect or plant.
In addition, the chemicals to be dispensed must be applied at or above a minimum application rate per unit area. It will also be appreciated that it is desirable to disperse the chemical such that the chemical will cover and effectively control as large an area as possible. Thus, for each chemical being applied, there is a desired range of droplet sizes and application rates.
Moreover, depending upon the particular use of the chemical, it is desirable to be able either to maintain a dispersion of a constant droplet size, or to maintain a dispersion of droplet sizes that vary within the chemical's legally permissible and effectiveness range. For example, when the liquid to be dispersed is a larvacide it is preferable to dispense larger sized droplets that will fall to the ground after they are suspended. If the liquid to be dispersed is an adultacide, smaller sized droplets are preferred so that the droplets will remain airborne thereby coming in contact with its targets.
In the past, it has been difficult to control or effect the size of the dispersed droplets because the droplet size is directly dependent upon the velocity of air delivered to the nozzle assembly. The air pressure thus produced remains constant because the air blowers engine can only be run at a steady constant output. Although it has been attempted to vary the output of the air blower engine, and hence the air pressure disclosed, this is not cost effective because it greatly reduces the engine life of the air blower.
Consequently, accepting the fact that the amount of air pressure developed is a constant, those skilled in the art have commonly employed several ways to vary the sizes of the dispersed droplets. It is generally recognized that in order to change a droplet size, either the flow rate of the liquid to be dispersed should be changed, and/or the air pressure used to disperse the liquid should be changed. For example, when the air pressure of the air, as it is received by the nozzle assembly, is held constant, and the flow rate at which the liquid to be dispensed is varied, low liquid flow rates will yield small droplets, and high liquid flow rates Will yield large droplets.
Another way to control the size of the disperse droplets is to maintain the delivery of the liquid to be dispensed by the nozzle assembly at a constant flow rate, and vary the air pressure of the air actually delivered to the nozzle assembly. In this type of apparatus, high air pressure yields smaller droplets, and low air pressure yields larger droplets. In the commonly owned and application Ser. No. 07/650,281, now abandoned, the amount of air actually delivered to the nozzle assembly has been controlled by reducing or "bleeding off" a portion of original generated air, through manually adjusting a pressure regulating assembly. However, that type of pressure regulating assembly will only bleed off air at a constant rate. Therefore, the generator will dispense an essentially constant size droplet.
Others in the art have attempted to vary the sizes of the dispensed droplets by providing a cluster or a plurality of nozzle assemblies that respectively dispense preset different sizes of droplets. A fog containing different size droplets can thus be produced if enough nozzle assemblies are used. However, this type of generator can be quite cumbersome and the number of different droplet sizes that it can produce is dependent upon the number of nozzle assemblies, used.